1. Field of the Invention
The present invention relates to a digital image forming apparatus, and particularly to a digital image forming apparatus in which an image is displayed digitally by controlling light modulation rate of a spatial modulation element on the basis of digital image data and the image is exposed onto a photosensitive material with modulated light.
2. Description of the Related Art
In a photographic printer in which a photographic print is prepared in such a manner that an image photographed to be recorded on a photographic film (for example, a negative film) is printed on a photosensitive material, usually, light is irradiated on the image recorded on the photographic film and the light transmitted through the image is exposed onto a photosensitive material such as a photographic printing paper. There has been recently proposed a photographic printer provided with, in addition to a first exposure section in which the light transmitted through an image on a photographic film is directly exposed onto a photosensitive material as described above, a second exposure section in which an index print image is exposed onto the photosensitive material.
The second exposure section of the above photographic printer is constructed in such a manner that images recorded on the photographic film are sequentially picked up by an image reading device and the thus obtained image data are stored in an image storage section. When index print images are formed on a photosensitive material, image data are read from the image storage section by several frames (by one row of frames) at a time and images of one row of frames are displayed on a liquid crystal panel based on the read image data, and further, light is irradiated on the liquid crystal panel and the light transmitted through the liquid crystal panel is exposed onto the photosensitive material.
When the above-described operation is repeated for each of a plurality of rows of frames, an index print is formed in which a plurality of film images on the photographic film is arranged in the form of a matrix. A user can easily effect retrieval of an image recorded in each of frames on the photographic film, or the like by referring to the index print.
In order to allow display of an image on the liquid crystal panel, a predetermined drive voltage is applied to each of a large number of liquid crystal cells which form the liquid crystal panel, so that the light transmissivity is adjusted for each of the liquid crystal cells. However, each of the liquid crystal cells includes a light non-controllable area which does not allow adjustment of light transmissivity due to the existence of wiring or the like, in addition to a light controllable area which allows adjustment of light transmissivity in accordance with the drive voltage. For this reason, when an image is formed in such a manner that the light transmitted through the liquid crystal panel is exposed onto the photosensitive material, a region on the photosensitive material corresponding to the light non-controllable area cannot be exposed, so that a systematic pattern of a certain fixed density (which is white on a paper) is produced within the formed image.
In order to prevent formation of the above-described pattern, a so-called pixel shift method has conventionally been carried out in which exposure is effected plural times with the liquid crystal panel being moved by a very small amount by piezoelectric elements or the like. In the pixel shift method, since an image is exposed with the liquid crystal panel being moved by very small amount, the light transmitted through the light controllable area of each of the liquid crystal cells can be irradiated on a region on the photosensitive material corresponding to the light non-controllable area, thereby making it possible to prevent formation of the above-described pattern.
However, the above-described conventional method requires expensive piezoelectric elements or the like which is used to allow high accurate movement of the liquid crystal panel by small bits, thereby resulting in complication of the structure of the apparatus and increase in cost thereof.